1. Field of the Invention
The present invention relates to a turbocharged internal combustion engine having reduced high speed emissions and, more particularly to an exhaust driven supercharger having turbine inlet pressure dependent control means for regulating the compressor pressure profile.
2. Description of the Relevant Art
Due to the broad speed range of automotive internal combustion engines, exhaust-gas driven superchargers, or turbochargers, must be regulated in order to achieve desired charge-air pressures over wide ranging engine speeds. Important factors in the design of such systems are cost, reliability and performance. As a result, the design should be simple. Conventional turbocharger systems supply boost to the engine at a progressively increasing rate until a maximum level is attained; normally in the area of peak engine torque, see curve "C" of FIG. 2. It is at this level of operation where the increase in charge density is most useful. Once maximum compressor pressure out, or boost is achieved, it is regulated to a constant value by a compressor pressure dependent wastegate. Maintenance of the compressor pressure at a constant value results in an increasing turbine pressure profile.
Conventional design turbochargers generally maintain maximum boost as engine speed increases along a declining torque curve, resulting in high mass air flow and high average peak cycle pressure during operating conditions which benefit little from these conditions. It is generally recognized that NOx emissions are related to total engine air flow during high speed, part-load conditions. Also, durability is closely related to the average peak cycle pressure that the engine is exposed to throughout its life. As the average peak cycle pressure is increased, engine component durability suffers.
In order to obtain the best compromise between engine emission performance, engine performance, and engine durability, it is desirable to apply turbocharger boost only during operating conditions that benefit from additional charge density.